This invention relates generally to correcting lens distortion.
A number of lens aberrations are specified by one coefficient in a series expansion. See M. Born and E. Wolf, Principles of Optics 6th Edition, Cambridge University Press (1998). These aberrations include spherical aberration, coma, astigmatism, curvature of field and distortion.
To varying degrees, camera lenses are subject to distortion. One class of such distortion errors gives rise to barrel and pincushion distortions. These distortions involve a bulging in the center of the image caused by irregularities in the lens manufacturing process. Barrel distortion has a negative distortion coefficient while pincushion distortion has a positive distortion coefficient. As a result of barrel distortion, a square or rectangular image bows outwardly to produce a resulting image which is barrel shaped. Thus, barrel distortion occurs when the coefficient of the appropriate series expansion is a nonzero value.
Digital cameras may include algorithms for correcting the barrel/pincushion distortion. Typically a third order warping transformation is used to determine the amount of curvature resulting from the distortion and to create a corrected image which closely models the actual exposed image. Generally, these techniques involve exposing a square or rectangular shape and determining the amount of curvature that results from the distortion. The third order warping transformation is used to convert the curved sides back to their square or rectangular shape. Through calibration, the distortion can be decreased.
However, with existing techniques, the accuracy may not be sufficient to enable two dimensional stitching. In addition, it may be difficult to accurately determine the amount of lens distortion. It may also be difficult to determine whether or not one has correctly located the optical axis of the lens. Thus, there is a continuing need for improved techniques for correcting for lens distortion.